How do you help your boyfriend become more mature?

I don't think that is possible. Men mature slower than we do and my Boyfriend has told me numerous times that he will never grow up. As far as seeing it your way, stop caring so much and "nagging" when he doesn't think something is serious. Men see the world, especially relationships a lot differently than we do. Maturity is something that comes with experience and the only way for him to treat something in a mature fashion is for him to realize that whatever it is may have a serious affect on him. I suggest you do this by explaining it to him and leaving it at that. As soon as he starts acting immature about something that you think is serious, tell him so and then go your separate ways for a couple hours or a day or two. Let him think about it. Men don't learn through listening, they learn when you show them.

You can't. That seems to be the biggest problem with relationships when you are young. Just be happy and deal with it for now. It gets better as you get older.